1. Field of Invention
The invention relates to a piezoelectric transducer and an ink ejector using a piezoelectric transducer.
2. Description of Related Art
A piezoelectric ink ejector has been conventionally proposed for a printhead. In a drop-on-demand ink ejector, a piezoelectric transducer deforms to change the volume of an ink channel containing ink. Ink in the ink channel is ejected from a nozzle when the volume is reduced, while ink is drawn into the ink channel when the volume is increased. Typically, a number of such ink ejecting mechanisms are disposed adjacent to each other, and ink is selectively ejected from an ink ejecting mechanism located in a particular position to form desired characters and graphics.
In a conventional piezoelectric ink ejector, one piezoelectric transducer is used for each ink ejecting mechanism. In this case, if a number of ink ejecting mechanisms are clustered to form an image over a wide range at high resolution, the ink ejector becomes complicated in structure and expensive to manufacture. In addition, it is hard to downsize each ejecting mechanism because the piezoelectric transducer cannot be made smaller due to machining constraints. Thus, the resolution is limited in such an ink ejector.
To address the forgoing problems, a single piezoelectric transducer disposed across a plurality of ink channels has recently been proposed for a piezoelectric ink ejector. A portion of the single piezoelectric transducer corresponding to a particular ejecting mechanism is locally deformed. Such a piezoelectric ink ejector is disclosed in U.S. Pat. No. 5,266,964. A piezoelectric ink ejector that has the same operation principle as that disclosed in the above patent is shown in FIGS. 23, 24. A piezoelectric ink ejector 401 includes a piezoelectric transducer 400, an ink channel forming member 60, and a spacer member 70, and a nozzle plate 90 having nozzles 80 connected to holes 71 formed in the spacer member 70.
The Piezoelectric transducer 400 is disposed across a plurality of ink channels 50 to change the volume of each ink channel 50. The piezoelectric transducer 400 is made by laminating a plurality of piezoelectric ceramic layers 410 while sandwiching spaced inner electrodes 430, 440 placed along each piezoelectric ceramic layer.
The piezoelectric ceramic layers 410 are polarized in the laminating direction, as shown by arrows P1. Each column of inner positive electrodes 430 is centered over each ink channel 50, and each column of inner grounded electrodes 440 is placed at either edge of each ink channel 50 (on the upper end face of the ink channel forming member 60).
When an ink droplet is ejected from an ink channel 50 based on a predetermined print data, a drive voltage is applied to the inner grounded electrodes 440, 440 at both edges of the ink channel 50 and to the inner positive electrodes 430 at the center. At this time, electrical fields are generated in the piezoelectric ceramic layers 410 (which form a piezoelectric transducer) symmetrically with respect to the inner positive electrodes 430 and perpendicular to the polarization directions, i.e. parallel to the inner positive electrodes, as shown by dashed arrows E1. As a result, two portions of the piezoelectric transducer on both sides of the inner positive electrodes 430 are deformed into parallelogram shapes by a shear effect, and the inner positive electrodes 430 are shifted upwardly in FIG. 23, thereby increasing the volume of the ink channel 50. At this time, ink is supplied from an ink source (not shown). Thereafter, when the application of the drive voltage is stopped, the deformed piezoelectric transducer returns to its original state. Thus, the volume of the ink channel 50 is reduced, and an ink droplet 81 is ejected from the ink channel 50 through the corresponding nozzle 80.
The ink ejector structured as described above is easy and inexpensive to manufacture and able to accomplish high-resolution printing.
However, in the above-described piezoelectric ink ejector, when the required ink droplet volume and the required ink ejecting velocity are fixed, the required drive voltage is determined by the spaces between inner positive electrodes 430 and their adjacent inner grounded electrodes 440, 440 provided for each ink channel 50. Thus, the drive voltage cannot be lowered as desired, resulting in an increase in the costs of a power source and a driving circuit board. In addition, when the drive voltage is fairly high, the polarization property of the piezoelectric transducer 400 tends to deteriorate due to the drive voltage applying direction and the polarization direction that are perpendicular to each other, which shortens the lifespan of the ink ejector.
When the spaces between inner positive electrodes 430 and their adjacent inner grounded electrodes 440, 440 provided for each ink channel 50 are decreased to lower the drive voltage, locally deformable areas of the piezoelectric transducer 400 are reduced, and the amount of change in the volume of ink in the ink channel 50 is also reduced. Because of such structural limitations, it is hard to decrease the drive voltage.
U.S. Pat. No. 6,174,051 and Japanese Laid-Open Patent Publication No. 10-58675 disclose another piezoelectric transducer, in which a piezoelectric ceramic layer that deforms in a shear mode is laminated on another piezoelectric ceramic layer that deforms in an expansion/contraction mode. The disclosed piezoelectric transducer deforms fairly effectively in combined modes. However, a need for a more effectively deformable piezoelectric transducer still exists.
The invention provides a piezoelectric transducer that can be effectively deformed with a low voltage and also provides an ink ejector that is driven with a low voltage, has high durability, and can reduce the costs of a power source and a driving circuit board.
According to one aspect of the invention, a piezoelectric transducer includes a piezoelectric ceramic member and a plurality of electrodes spaced along the piezoelectric ceramic member. The plurality of electrodes includes a first set of electrodes defining therebetween at least one first area and a second set of electrodes split by the at least one first area and defining a second area on each side of the at least first area. The two second areas are polarized substantially perpendicular to opposing directions of electrodes of the second set. Upon application of a drive voltage to the first and second sets of electrodes, an electric field is generated in each of the two second areas substantially perpendicular to the polarization direction, and each of the two second areas is obliquely deformed by a piezoelectric shear effect to unidirectionally shift the at least one first area. At the same time, the at least one first area is deformed to increase a space created between the deformed two second areas.
When the above-described piezoelectric transducer is placed across a plurality of ink channels, a first set of electrodes and a second set of electrodes are provided for each ink channel. At least one first area is substantially centered over each ink channel, and two second areas are located near both edges of each ink channel. When at least one first area and two second areas over a selected ink channel are deformed as described above, the volume of the ink channel is changed, causing ink ejection from a nozzle of the selected ink channel.